Environmental contaminants contained in emissions from coal-fired and oil-fired power plants are a major environmental concern. Particulate matter (e.g., fly ash), nitrates, sulfates, and mercury emissions are restricted because these emissions can yield for example, acid rain and, serious neurotoxic effects. The removal of particulate matter has be addressed through, for example, the installation of baghouses, cyclone separators, or cyclone separators with baghouse filters in the flue gas ducts. The removal of nitrates and sulfates has be addressed through the addition of lime (calcium oxides and/or hydroxides) to the flue gas and the collection of the lime reaction product (e.g., CaSO4) with the particulate matter. The removal of mercury can be addressed by absorption with a mercury absorbent material. Unfortunately, the mercury absorbent materials and lime are often chemically incompatible and/or the mercury absorbent material is fiscally incompatible with the collection and disposal of the particulate material which is often sold into the concrete industry.
The most common method for reduction of mercury emissions from coal-fired and oil-fired power plants is the injection of powdered, activated carbon into the flue gas stream. The activated carbon is a high surface area material that provides for the adsorption of the mercury and the agglomeration of the particle bound mercury. The disadvantage of adding activated carbon into the flue gas stream is the retention of the activated carbon in the fly ash waste stream. Fly ash from coal-fired power plants if often added to concrete, where the presence of the activated carbon adversely affects performance, thereby making the inclusion of the carbon fiscally incompatible with the flue gas scrubbing process.
Another method for reducing Hg emissions is through the addition of chemical species that react with mercury to chem-adsorb the elemental Hg and oxidized Hg. One class of materials capable of chemically reacting with Hg are metal sulfides. U.S. Pat. No. 6,719,828 teaches the preparation of layered sorbents such as clays with metal sulfide between the clay layers. The method used to prepare the layered sorbents is based on an ion exchange process, which limits the selection of substrates to only those having high ion exchange capacity. In addition, the disclosed ion exchange is time-consuming, involving several wet process steps significantly impairing the reproducibility, performance, scalability, equipment requirements, and cost of the sorbent. The process of making a sorbent, in accordance with the teachings of U.S. Pat. No. 6,719,828, involves swelling a clay in an acidified solution, introducing a metal salt solution to exchange metal ions between the layers of the clay, filtering the ion exchanged clay, redispersing the clay in solution, sulfidating the clay by adding a sulfide solution, and finally filtering and drying the material. Another shortcoming of the process disclosed in U.S. Pat. No. 6,719,828 is the environmental liability of the by-products of the ion exchange process, i.e., the waste solutions of metal ions and the generated hydrogen sulfide.
U.S. Pat. No. 7,578,869 teaches the preparation of metal sulfide/bentonite clay composites for the removal of mercury from flue gas streams. The application teaches two methods, an “incipient wetness” process and a solid-state reactive grinding process, to prepare the composites. The processes are similar in that a copper salt is mixed with a bentonite clay and then a sulfide salt is added. The processes differ in the method of addition of the sulfide salt. In the first method the sulfide salt is added through an “incipient wetness” procedure where the sulfide salt is dissolved in water and added to the copper/clay mixture as an aqueous solution; in the second method the sulfide salt is added through a “solid-state reactive grinding” process where the sulfide salt hydrate is ground with the hydrated copper/clay mixture. The patent further teaches that the incipient wetness and solid-state grinding methods differ from the “wet” method of U.S. Pat. No. 6,719,828 because there is no ion-exchange of the copper ion for the exchangeable cationic ions of the bentonite clay.
The composite nature of the materials produced in the patent are supported by powder X-ray diffraction spectra that provide evidence of the formation of covellite (CuS), the same copper sulfide prepared in U.S. Pat. No. 6,719,828.
While U.S. Pat. No. 7,578,869 disclaims ion exchange, copper salts and bentonite clays readily and easily ion exchange to yield very stable copper/clay compositions. See e.g., Ding, Z. and R. L. Frost, “Thermal study of copper adsorption on montmorillonites.” Thermochimica Acta, 2004, 416, 11-16. Analytical analysis of these compositions confirms both interlayer ion-exchange (intercalation) and edge adsorption of the copper salt. See e.g., El-Batouti et al., “Kinetics and thermodynamics studies of copper exchange on Na-montmorillonite clay mineral” J. Colloid and Interface Sci. 2003, 259, 223-227.
The disadvantage of these copper-based mercury absorbent materials is that these materials are known to react with lime, for example, yielding copper metal and calcium sulfide materials. See e.g., Habashi et al., Metallurgical Transactions, 1973, 4, 1865. These reaction products are unsuccessful in the absorption of mercury from the flue gas. Therefore, the use of the copper based mercury absorbent materials have been physically separated from the use of lime in the flue gas scrubbing process.
There is still an ongoing need to provide improved pollution control sorbents and methods of their manufacture and it is desirable to provide mercury sorbents that are compatible with lime.